Using a 555, this configuration appears elsewhere on this site. This is from American documentation from the 90s. The indicator can be an LED or some device controlled by a relay.
This calibrated circuit was found in documentation from the 1980s. It makes use of an integrated circuit...
The figure shows how to add a Darlington stage to an operational amplifier in order to obtain a...
This circuit has two stable states such that at each instant only one SCR leads. The state change is...